DESCRIPTION (adapted from the applicant s abstract): Whether a particular antigen will induce a tolerogenic or immunogenic response is determined by the context in which the antigen is encountered rather than by the antigen itself. T cell clones that engage their T cell receptor (TCR) in the absence of co- stimulation not only fail to produce IL-2 and proliferate, but are hyporesponsive upon subsequent full re-challenge; they are anergic. Current data suggests that it is not co-stimulation that prevents the induction of anergy, but rather TCR engagement in the absence of IL-2 induced cell cycle progression. Rapamycin, which inhibits cell cycle progression in G1, induces anergy in the presence of TCR engagement and co-stimulation, whereas hydroxyurea, which inhibits the cell cycle in S phase does not induce anergy under similar conditions. It is hypothesized that TCR engagement leads to the up-regulation of the negative regulatory factors that mediate anergy, and that IL-2 induced cell cycle progression from G1 to S phase serves to inactivate/degrade these factors. By examining the cyclin-dependent cascade, this application seeks to define the biochemical events that take place between G1 and S phase that are responsible for determining the anergic/non- anergic fate of an activated T cell. Once identified, candidate genes will be over-expressed in T cell clones in order to determine their effect on T cell activation. Although the precise mediators of anergy have yet to be elucidated, one mechanism for the maintenance of anergy appears to be the formation of a transcriptional inhibitory complex at the IL-2 promoter. This complex, which binds to the 180 site of the IL-2 promoter, contains a cAMP- responsive element binding protein (CREB)/cAMP-responsive element modulator (CREM) heterodimer and appears to be facilitated by the binding of the high mobility group protein HMG (I) Y. It is hypothesized that the CREB/CREM complex mediates the inhibition of IL-2 by recruiting co-repressors to the IL- 2 promoter. Initial experiments will determine if the over-expression of p300, a co-activator of IL-2 transcription, has the ability to overcome this transcriptional repression. Second, experiments will be performed to try to identify these co-repressors using DNA affinity precipitation. In addition, the potential role of histone acetylation as well as histone deacetylase activity in mediating this repression will be explored. Ultimately, understanding the pathways that mediate T cell inhibition might help in the generation of more specific immunosupressive agents for preventing allograft rejection and treating autoimmunity, as well as designing agents that inhibit these pathways in order to promote tumor immunity.